1. Field of the Invention
The present invention relates to digital image sensors, and, more specifically, to a color saturation adjustment method for image sensors and digital cameras.
2. Description of the Related Art
Saturation is an important visual aspect when viewing color images and video. When capturing color images by a digital camera, captured spectral data must be processed in order to match the original scene viewed by the human eye. Considerable saturation boost has been used to process the data to match the original scene. Saturation is the ratio of the dominant wavelength to other wavelengths in a color.
It is desirable to adjust saturation of colors in digital images without changing other important visual aspects, such as, for example, the perceived hue of the color, or the uniformity across different hues in the image.
There are several known methods and color models for adjusting saturation without disrupting other visual characteristics of a digital color image. For example, the HSV (Hue, Saturation, Value) model is a well-known color model in which the saturation attribute is independently adjustable. HLS (Hue, Lightness, Saturation) is another closely related model. While these models may be effectively utilized to adjust saturation in digital color images, the cost and complexity of adjusting saturation according to these and other known methods is substantial. For example, non-linear behavior such as perceived hue changes and non-uniformity exists when saturation is adjusted substantially. Other color models such as the 1931 standard observer model adopted by the Commission Internationale de l'Eclairage (CIE) based on a model of human rods and cones, also exhibit non-uniformity and/or non-constant hue behavior. The same is true of numerous CIE derivative models such as LUV and L*a*b.
In color television systems, “YCbCr” color difference space is used to encode color information. It also provides a convenient color space for hue and saturation adjustments. This technique, however, also suffers from the non-uniformity and hue changes when saturation is increased.
As the shortcomings of these known saturation adjustment methods demonstrate, human vision is a highly adaptive and complex system that cannot be accurately mapped to a simple and regular geometric space for all possible viewing conditions. Adapted white point and color responses will change according to the viewing conditions.
In contrast to the known electronic image sensors, the three types of color receptors in the human eye—long-, middle-, and short-wavelength cones (LMS)—have been found to exhibit significant overlap in spectral response. As a consequence of this overlapping spectral response, the hue-discrimination response of the human eye is highly non-linear, with peak sensitivity occurring near certain wavelengths. Furthermore, each LMS receptor has independent gain control.
It is desirable to have an imaging system having saturation adjustment that operates as close as possible to the LMS cone spectral response space to thereby more closely replicate human vision.